The present application is based on and claims priority from Japanese Patent Applications 2000-239918 filed Aug. 8, 2000 and 2001-144937, filed May 15, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a vehicle AC generator and a voltage regulator thereof.
2. Description of the Related Art
Although a field coil is not supplied with field current, an armature winding of a vehicle AC generator generates a small output voltage signal if a rotor of the generator rotates because of residual magnetic flux of pole cores mounted in the rotor. U.S. Pat. No. 5,376,876 discloses a voltage regulator in which terminal voltage of an armature winding of an AC generator is compare with a staircase wave-form to detect rotation of the rotor.
If leak current flows from the outside of a high DC potential into the armature winding while the field coil is not supplied with field current, the output terminal of the armature winding is biased by the high DC potential. Accordingly, the output voltage of the armature winding is too high to be compared with the staircase wave-form, and the comparator does not provide a normal output signal as long as the leak current flows into the armature winding.
In order to solve the above problem, WO99/07064 proposes an AC generator in which the output voltage of an armature winding is compared with a variable threshold value of a window comparator to detect rotation of a rotor.
However, if leak current flows into the armature winding from the outside of a high DC voltage, it is impossible to know the level of the DC voltage because contact resistance is not fixed and widely changes. It is necessary, but impossible to provide an infinite number of threshold levels to detect a signal of the leak current.
Mechanism of the DC biasing when the leak current flows is discussed below with reference to FIGS. 8-14.
Firstly, FIG. 8 schematically shows a case in which leak current flows into a Z-phase winding for generating phase voltage Pz that is not used as a rotation signal. If contact resistance between the output terminal of the Z-phase winding and +B potential of, for example a positive cooling fin of the full-wave rectifier is R1 and contact resistance between the output terminal of the Z-phase winding and a ground, for example a housing is R2, the contact resistances R1 and R2 changes widely according to foreign material such as salt water, mud water, or its dried product, crystals, rust or the like.
FIG. 9 shows an equivalent circuit diagram when leak current flows. The potential or terminal voltage Pz of the terminal of the Z-phase winding is expressed as follows: Vbattxc2x7R2/(R1+R2). Since the amounts of the resistances R1 and R2 are not fixed, the potential Pz of the terminal is not fixed.
When an engine is started and the rotor of the AC generator is rotated, the terminal of the Z-phase winding is applied a voltage that alternates and changes according to rotation speed of the rotor from a value expressed by Vbattxc2x7R2/(R1+R2). (as shown in FIG. 10).
In WO99/07064, although the threshold value of the window comparator is variable, only a limited number of threshold values are provided. It takes a time to detect all the variable DC bias voltage signals, and it is too late when the signal is found to correspond to one of the threshold values that were provided. Therefore, it is impossible to make the AC generator generate power concurrently when the engine starts.
Further, if leak current flows into a Y-phase winding for generating phase voltage Py that is detected and used for the rotation signal, as shown in FIG. 11 (or equivalent circuit is shown in FIG. 12), the potential thereof is fixed to the value expressed by Vbattxc2x7R2/(R1+R2). As a result, it is impossible to detect the potential of other X and Z-phase windings unless the voltage Px or Pz becomes higher than Vbatt or lower than the ground voltage. That is, if the generated voltage is lower than the DC bias voltage, the generated voltage levels of the X-phase and Z-phase windings are masked by the DC bias voltage (as indicated by broken lines in FIG. 13). Therefore, it takes a delay time xcfx84 to detect the generated voltage. (as shown in FIG. 13).
The delay time xcfx84 becomes the longest when the DC bias voltage becomes as high as a half of the terminal voltage of the vehicle battery, as hereafter described in more detail.
If the DC bias voltage becomes lower than a half of the terminal voltage of the vehicle battery, in other words, if the contact resistance R1 is larger than the contact resistance R2, the voltage Px or Pz of the X-phase or Z-phase winding alternately changes, with the DC bias voltage applied to the Y-phase winding being the center of the amplitude. If the voltage of the bias voltage is lower than a half of the battery terminal voltage, the phase voltages Px and Pz become the ground voltage faster than they become the battery terminal voltage. If the Z-phase voltage Pz becomes the ground voltage, a diode connected to the Z-phase winding passes signal current i1, so that the signal voltage becomes higher than the bias voltage. If the phase voltage Px becomes the ground voltage, a diode connected to the X-phase winding, likewise, passes signal current, as shown in FIG. 13A.
On the other hand, if the DC bias voltage is higher than a half of the terminal voltage of the vehicle battery, in other words, if the contact resistance R1 is smaller than the contact resistance R2, the phase voltage Px or Pz alternately changes, with the bias voltage being the center of the amplitude. If the bias voltage is higher than a half of the battery terminal voltage, the phase voltages Px and Pz become the battery terminal voltage faster than they become the ground voltage. If the phase voltage Px becomes the battery terminal voltage, a diode connected to the X-phase winding passes signal current i2, so that the signal voltage becomes lower than the bias voltage, as shown in FIG. 14A.
Therefore, if the bias voltage applied to the Y-phase winding becomes as high as a half of the battery terminal voltage due to leak current, it takes the longest time for the phase voltages Px and Pz of the X-phase and Y-phase windings to become 0 V or the battery terminal voltage. That is, if an amount of the leak current is very large, it takes a long time to generate the rotation signal to be detected.
JP-A-3-215200 and PCT National Publication 8-503308 disclose control circuits in which terminal voltages of two phase windings are detected to thereby detect leak current. However, it is necessary to insert two terminals of the armature winding into the control circuit, which may become complicated in structure and increase in the number of joints of wires, resulting in lowering the reliability of the control circuit.
In the control circuit disclosed in JP-A-3-215200, a voltage difference between the terminals of two phase windings is detected in a voltage level that floats from the ground. Therefore, it is necessary to provide a specific comparator with a complicated supply source if such a floating voltage is to be compared with a reference voltage. Moreover, when the alternator starts generation, excessively high voltage may be applied to the comparator. Therefore, it is necessary to provide various protecting means for the comparator, resulting in a large size of the control circuit.
The present invention is to provide a voltage regulator of a vehicle AC generator that can surely detect a voltage signal from a terminal of a multi-phase armature winding.
In order to attain the above object, a voltage regulator of a vehicle AC generator according to a main feature of the invention includes means for detecting phase voltage of an armature winding, and a comparator for comparing phase voltage with a variable threshold level signal that varies according to the phase voltage. Even if leak current takes place and DC bias voltage is applied to the armature winding, a binary pulse signal can be surely formed so that the start of the rotation can be surely detected without delay. Accordingly, no exclusive signal line for detecting turning-on of a key switch is necessary.
A pulse detecting means for detecting the number of the pulses of the output signal of the comparator or frequency thereof may be provided. The AC generator is operated if the pulse detecting means detects the number or frequency to be the same as or larger than a predetermined level. Therefore, the signal can be processed digitally at a high speed, so that the rotation can be detected without delay by a compact integrated circuit.
The variable threshold level may be an electric signal that corresponds to one of the maximum voltage and the minimum voltage of the phase voltage. The variable threshold level can be formed by a positive peak hold circuit or a negative peak hold circuit.
The variable threshold level signal may be an electric signal that corresponds to any value between the maximum voltage and the minimum voltage of the phase voltage. In this case, the variable threshold value signal can be formed by combining a positive or negative peak hold circuit and a voltage dividing resistors. Further, the DC bias voltage due to the leak current can be detected at any case.
The variable threshold level signal may cross the phase voltage twice in a cycle of the phase voltage while the rotor is rotating. Therefore, the digitalized binary output pulse signal of the comparator can be surely formed. The variable threshold level signal may be a signal corresponds to a mean value of the phase voltage. Therefore, the variable threshold value signal can be formed easily by an integration circuit or the like.
The frequency of the output pulse of the comparator may be equal to the frequency of the phase voltage. Therefore, the frequency of the phase voltage, which is caused by rotation of the rotor, can be accurately detected. In other words, the rotation speed of the rotor can be accurately detected.
The voltage regulator may further includes a second comparator for comparing the phase voltage with a reference value, and the field current is supplied to the field coil for a predetermined period to increase the phase voltage when the second comparator reverses the output signal thereof.
Therefore, even if a small signal generated by residual magnetic flux can not be detected due to the leak current appearing in the phase winding that is detected, the signal is amplified by supplying field current to have a voltage level sufficient to be detected.
The phase voltage detecting means may include a first resistor connected between the armature winding and the negative terminal of a vehicle battery, a second resistor that has much lower resistance than the first resistor and connected, together with a switching means, in parallel with the first resistor. If the switching means is closed especially in a case that leak current flows in the phase winding whose terminal voltage is detected as the rotation signal, the terminal voltage can be lowered. Therefore, the delay time until a signal is increased to an amount to be detected can be reduced.
Further, if the detected phase voltage becomes higher than a predetermined voltage level, the switching means is opened. Therefore, power generated in the armature winding is prevented from being wasted by the second resistor.
The switching means may be a MOSFET that is easy to mount in a circuit. Moreover, the signal current can be supplied not only from the winding to the negative terminal of the battery, but also from the negative terminal of the battery to the winding. Therefore, the rotation can be detected even if the contact resistance varies widely.
The predetermined period may be longer than a period during which the field coil is supplied with field current so that the output voltage of the armature winding becomes as high as about a half of a nominal vehicle battery voltage. That is, the field current is supplied until the signal is generated even in the case that leak current takes place and the output voltage of the armature winding is fixed to be as high as a half of a nominal voltage of the vehicle battery. Therefore, the signal can be surely detected. Accordingly, the rotation of the rotor can be detected whenever any leak current flows.
The voltage regulator may cut the field current for a period after the field current is supplied to the field coil and before the field current is supplied to the field coil for a next period. Therefore, the field current is prevented from being continuously supplied as long as the leak current flows.